User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.83 Safari/537.36

Steps to reproduce:

81.0b2 (OS X):
Saved an attachment from an inbox message.

Actual results:

Alert notified that message had been saved and would now be permanently deleted from the message. I said OK. But attachment remained in the message, and now the “Delete” item is disabled in the attachment menu, preventing it from being deleted.

Expected results:

Saving the attachment does not delete it. Deletion remains possible after an attachment is saved.

Both delete and detach work for me on Mac with 81.0b3

What do you see in Tools > developer tools > error console?

Jonathan, I bet you have (accidentally?) clicked "Detach" instead of delete. That will remove the attachment from the message but immediately replace it with a link to the saved file on your harddisk. Unfortunately, even after bug 1345167, there's pretty much no visual difference between the link to the detached file in your attachment pane and the previous real attachment. So indeed it looks as if the attachment is still there. Please hover the attachment with mouse, and it now looks like a hyperlink, right? Also, in status bar, you'll see the path to the file on your harddisk. Essentially, this is bug 292385 which isn't really resolved yet by bug 1345167 to which it was duplicated.